In a 9" x 13" baking dish coated with cooking spray, spread about 1/2 cup marinara sauce over the bottom.

In a large bowl, add ricotta, Asiago, onion powder, oregano, crushed red pepper, salt, black pepper, spinach, egg yolk and garlic - stir until well combined. Scoop the mixture into a large zip-loc bag and push the mixture down to one corner. Remove any excess air from the bag and seal to close - snip one end of the corner that the mixture is at and fill each cooked pasta shell with roughly 1 1/2 tablespoons worth of the ricotta filling.

Place the stuffed shells in a single layer on top of the marinara-coated baking dish. Spoon the remaining marinara sauce evenly over the shells. Cover and bake until thoroughly heated though, about 30 minutes. Remove and let sit for about 5 minutes before serving. Serve with a sprinkling of Asiago cheese if desired.